1. Field of the Invention
The present invention relates to a zoom lens and an image projection apparatus using it and, more particularly, to an image projection apparatus suited for a projector for enlarging and projecting, for example, image information of a display member onto a screen and an optical device such as a video camera, film camera, or digital camera for forming image information on an image pickup means such as a film or CCD.
The present invention also relates to a compact zoom lens whose pupil matching property is kept good with respect to an illumination system when applied to a projector and which is suited for a high-resolution mobile liquid crystal projector, in particular.
2. Description of the Related Art
Conventionally, various types of liquid crystal projectors (projection apparatuses) have been proposed, which are designed to project images based on display elements such as liquid crystal display elements onto screen surfaces by using the display elements.
Liquid crystal projectors, in particular, have been widely used as apparatuses for projecting images formed by Personal Computers and the like on large screens for conferences, presentations, and the like.
The following have recently been required for such liquid crystal projectors:
In a three-panel projector using three liquid crystal display elements of three colors, i.e., R, G, and B, a mount space for a dichroic prism serving as a color synthesis means for synthesizing chromatic light beams passing through the liquid crystal display elements and an optical element such as a polarizing plate must be located between the liquid crystal display elements and a projection lens, and hence a predetermined back focal length is required.
To minimize the influence of the angle dependence of a color synthesis film used for the color synthesis means or to ensure a good pupil matching property with respect to the illumination system, a so-called telecentric optical system must be used, in which the pupil on the liquid crystal display element (reduction) side is located at infinity.
If characters and the like look double when images (pictures) based on the liquid crystal display elements of three colors are synthesized and projected on the screen, a sense of resolution and quality deteriorate. In order to prevent this, pixels of the respective colors must be properly overlaid on each other throughout the entire screen. For this purpose, color shift (chromatic aberration of magnification) caused by the projection lens must be properly corrected throughout the entire visible region (the lens system must be an apochromat).
In order to prevent a contour portion of a projected image from distorting undesirably, distortion aberration must be corrected to xe2x88x922% (barrel distortion on the reduction side) or less (if an abrupt change in distortion aberration at the peripheral and intermediate portions of an image remains, the image quality undesirably deteriorates).
In consideration of the mobility of a projector equipped with a compact panel, there are needs for reductions in apparatus size and weight as well needs for increases in the luminance of a screen and the resolution of images.
Conventionally, various types of zoom lenses have been proposed as projection lenses for liquid crystal projectors.
Of these zoom lenses, a retrofocus type zoom lens in which a lens unit having negative refractive power serves as a front lens unit is widely used for optical devices such as video cameras, film cameras, and digital cameras as an image taking lens for wide view angle.
For example, Japanese Patent Publication No. 6-56453 discloses a zoom lens whose overall lens length remains unchanged in magnification changing operation, which is comprised of lens units respectively having negative refractive power, positive refractive power, negative refractive power, negative refractive power, and positive refractive power sequentially arranged from the enlargement side. When the magnification is to be changed from the wide angle end to the telephoto end, the second lens unit moves to the enlargement side, the third lens unit moves to the reduction side, and the fourth lens unit moves to the enlargement side so as to draw a convex locus.
In a one- or three-panel color liquid crystal projector, in particular, which is designed to project a display image based on a liquid crystal display member through a projection lens in enlarging or projecting the display image onto the screen, the following are required for the projection lens:
(A-1) To eliminate the influences of the alignment characteristics of a liquid crystal or the angle dependence of a color synthesis dichroic mirror in synthesizing a plurality of chromatic light beams, a so-called telecentric optical system in which the exit pupil is located at infinity must be used.
(A-2) To ensure a space for the color synthesis element interposed between the display member and the projection lens, the back focus length must be large.
(A-3) The projection lens needs to be small in size and weight and have high optical performance.
In the zoom lens disclosed in Japanese Patent Publication No. 6-56453, since the synthetic refractive power of the first to third lens units is negative, and incident light diverges and strikes the fourth lens unit, the positive refractive power of the subsequent lens units (on the reduction side) including the fourth lens unit needs to be high. For this reason, the asymmetry of the refractive power arrangement unique to a retrofocus lens is undesirably high in terms of aberration correction. It is difficult to realize a sufficiently compact optical system.
In addition, if the zoom lens disclosed in the above reference is applied as a projection lens for a mobile liquid crystal projector, the overall lens system is large. Therefore, a satisfactory result cannot always be obtained.
It is an object of the present invention to provide a zoom lens suited for a liquid crystal projector, which reduces the size of the overall lens system, properly maintains telecentric conditions throughout the entire magnification change range, properly corrects aberrations accompanying magnification changing operation, and has good optical performance throughout the entire screen, and an image projection apparatus using the zoom lens.
For achieving such object, one aspect of the present invention is a zoom lens comprising:
a first lens unit having negative refractive power, a second lens unit having positive refractive power, a stop, a third lens unit having negative refractive power, a fourth lens unit having negative refractive power, and a fifth lens unit having positive refractive power sequentially arranged from an enlargement side, each of lens units have one or a plurality of lenses,
wherein both refractive power on the enlargement side from the stop and refractive power on a reduction side from the stop are positive.
Further aspect of the present invention is a zoom lens comprising:
a first lens unit having negative refractive power;
a second lens unit having positive refractive power;
a third lens unit having negative refractive power;
a fourth lens unit having negative refractive power; and
a fifth lens unit having positive refractive power, the first to fifth lens units being sequentially arranged from an enlargement side,
wherein said third lens unit moves from a reduction side to the enlargement side when a magnification is changed from a wide angle end to a telephoto end.
Further aspect of the present invention is a zoom lens comprising:
a first lens unit having negative refractive power;
a second lens unit having positive refractive power;
a third lens unit having negative refractive power;
a fourth lens unit having negative refractive power; and
a fifth lens unit having positive refractive power, the first to fifth lens units being sequentially arranged from an enlargement side,
wherein when a magnification is changed from a wide angle end to a telephoto end, the first and fifth lens units are fixed, all the second, third, and fourth lens units move from a reduction side to the enlargement side, and letting xcex22w and xcex23w be imaging magnifications of the second and third lens units at the wide angle end, f5 be a focal length of the fifth lens unit, and fw be a focal length of an overall system at the wide angle end,
0.25 less than xcex22w less than 1.0 
1.0 less than xcex23w less than 5.0 
1.2 less than f5/fw less than 2.2 
are satisfied.
Further aspect of the present invention is an image projection apparatus comprising:
at least one image display element; and
aforementioned zoom lens, which projects an original projection image formed by the image display element onto a projection screen.
Further aspect of the present invention is an optical device for forming image information on image pickup means by using aforementioned zoom lens.